The invention relates to zone purification of silicon with reactive plasma heating and an object thereof is boron removal. The invention is particularly, though not exclusively, suitable for use in the manufacture of silicon for photovoltaic cells from commercially available silicon.
For silicon to be usable for photovoltaic cells, its content of several elements must be below very low thresholds. In particular, the percentage of elements termed electron killers (V, Cr, Ti, Zr, Na, Cu for example) must typically be less than 5 parts per billion. The percentage of neutral elements (particularly Fe, Ni, Mg, C, Mn) must not exceed about 1 ppm. Finally, the percentage of doping elements formed by impurities, typically boron and aluminium for P-type silicon, must be reduced to an extremely low value for later accurate control of the final content in doping elements.
Processes are known for purifying silicon by zone melting in which a zone of a bar of silicon to be refined is molten by directing onto this zone a jet of hot plasma obtained by high frequency excitation. The plasma jet and the bar are moved, with respect to one another, so as to cause the melted zone to travel along the bar once or more times. In French Pat. No. 2,438,499, there is described a process using a plasma formed from a plasma-producing gas, particularly argon, and a low percentage of hydrogen. Hydrogen has a reactivity which promotes the removal of certain impurities. But the removal of boron remains difficult with this process, as with other prior art zone purification processes, since boron and silicon form very stable compounds such as Si.sub.x B.sub.y. Moreover, boron has a theoretical partition coefficient in silicon between the liquid phase and the solid phase close to unity.
It is an object of the invention to improve the prior art processes and apparatuses for purifying silicon; it is a more specific object to provide a process which effectively removes boron.
A process according to the invention comprises the step of forming a plasma from a mixture of a plasma-producing gas, such as argon or helium, and oxygen, the amount of oxygen being low enough for avoiding appreciable oxidation of silicon.
That process is in direct contradiction with generally accepted ideas, according to which the presence of oxygen in the plasma is to be avoided; it was thought that oxygen, even in very small amounts, leads to troublesome oxidization of the silicon. It has been found that silicon oxidization may be avoided if the amount of oxygen contained in the plasma is adjusted to an appropriate value, which may typically be between 100 and 300 times the boron content. For the amounts of boron contained in currently available industrial silicon, the oxygen content of the plasma then remains sufficiently low to avoid oxidization of the silicon. As a general rule, the oxygen content will typically be between 0.005 and 0.1%, depending on the initial amount of boron contained in the silicon. Several successive passes under the plasma jet, possibly containing different percentages of oxygen, may be required to arrive at a sufficiently low boron content.
The favorable results obtained by the invention may probably be attributed to the transformation of the boron into borosilicate; there is segregation due to movement toward the surface of the bar and the tail end thereof. Borate, is transformed by decomposition of Si.sub.x B.sub.y into volatile compounds of B.sub.2 O.sub.3 type, which are outgassed from the surface of the silicon ingot and at the tail-end thereof. Complementary removal may be carried out by acid attack, after zone purification or after each pass. A mixture such as of hydrofluoric acid and nitric acid, may be used, according to a known technique.
The addition of oxygen to the plasma does not exclude simultaneous addition of a percentage of hydrogen which will be typically of about 1% so as to take advantage of the reactivity of hydrogen and the attendant temperature increase, which results in removal of other impurities.
On the other hand, it will be generally desirable to avoid nitrogen which would cause nitridation of the silicon.
The process of the invention may be carried out in an apparatus which comprises a conventional boat containing the bar to be processed and means for moving the boat and a plasma gun relative to each other. The plasma gun will be associated with means for supplying it with a mixture of plasma-producing gas and an adjustable amount of oxygen, for example by means of a controlled leak valve. The boat will be provided with cooling means, so that melting takes place in a "self-crucible"; then no impurity from the boat is introduced into the bar during purification.
The plasma may preferably be generated by a purely inductive method without any electrode which might introduce impurities into the plasma.
The invention will be better understood from the following description of a particular embodiment of the invention, given by way of example.